The present invention relates to a code division multiple access communications receiver.
Code division multiple access (hereinafter, CDMA) is undergoing intensive development as a technique for the efficient utilization of bandwidth in mobile communications systems. CDMA uses a spectrum spreading process to enable multiple signals to share the same frequency band. A desired signal is extracted from the shared band by a despreading process.
In a direct sequence CDMA system, the different signals are distinguished by the use of different spreading codes. When a signal is extracted, the extracted signal usually contains interference from other signals, due to imperfect orthogonality of the spreading codes and other factors.
A known method of canceling this so-called co-channel interference estimates the signal received from each transmitting station on each transmission path, and subtracts, from the combined received signal, the signals estimated to have been received from stations other than the desired station. The subtractions may be performed one after another in a serial manner, or they may be performed in a parallel manner after all signals have been estimated. The entire process may be repeated in two or more stages.
Besides canceling interference, it is also necessary to estimate and correct for communication channel effects. To aid in channel estimation, some CDMA systems have each station transmit a known signal, referred to as a pilot signal or training signal, in addition to the signal containing the unknown information the station is seeking to communicate. The pilot signal or training signal may be transmitted continuously, or at regular intervals, or only at the beginning of communication, or at the beginning of communication and at other times as necessary. The term `training signal` will be used below to cover all of these modes of transmission.
Aside from having known content, the training signal is transmitted like an information signal, although with a different spreading code. Accordingly, training signals also cause co-channel interference, which must be canceled in the receiver. Conventional CDMA receivers have canceled training-signal interference in a single operation, prior to the cancellation of interference due to the information signals.
This conventional method of canceling training-signal interference would be satisfactory if the received form of each training signal could be accurately predicted, but varying channel effects make accurate prediction difficult, and the conventional method usually leaves a certain amount of residual training-signal interference uncanceled. All information signals extracted from the received signal are affected by this residual training-signal interference, so residual training-signal interference becomes a significant factor limiting the number of stations that can be accommodated in the shared frequency band. In a cellular communication system, the cell capacity is limited.